<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0,user-scalable=no">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        .demo{
            width:50vw;
            height:200px;
            background-color: pink;

            /* 100px量出来 -> 到底要写多少vw ? */
        }
    </style>
</head>
<body>
    <div class="demo"></div>
</body>
</html>